Recent Studies on the Application of Enzymes in the Synthesis of Polyesters
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RECENT STUDIES ON THE APPLICATION OF ENZYMES IN THE SYNTHESIS OF POLYESTERS CARY J. MORROW, EUGENIA M. BRAZWELL, DIANELA FILOS, JUANITA MERCURE, ROSEMARY ROMERO, ANDJ. SHIELD WALLACE University of New Mexico, Department of Chemistry, Albuquerque, NM 87131 ABSTRACT Enzyme-catalyzed preparation of polymers offers several potentially valuable advantages over the usual polymerization procedures. This paper summarizes our successful use of lipase-catalyzed polycondensations to prepare both a series of achiral [AA-BB]. polyesters from bis(2,2,2trichloroethyl) alkanedioates and diols and of an optically active, epoxy-substituted polyester having a stereochemical purity estimated to be greater than 96%, from racemic bis(2,2,2-trichloroethyl) trans3,4-epoxyhexanedioate and 1,4-butanediol. All of the reactions were carried out at ambient temperature in anhydrous, low to intermediate polarity, organic solvents such as ether, THF, 2-ethoxyethyl ether, dibenzyl ether, o-dichlorobenzene, or methylene choride, using porcine pancreatic lipase (PPL) as the catalyst. The molecular weight achieved by the polycondensation is limited by accumulation of the trihaloethanol that forms as the reaction progresses, probably because it frees enzyme-bound water permitting hydrolysis of the polymer to occur. This problem has been alleviated by using a high boiling solvent and removing the alcohol by placing the re'action mixture under vacuum. INTRODUCTION During the past four years, research arising from the pioneering work of Klibanov and coworkers [1] has led to enzyme-catalyzed reactions in anhydrous, organic media taking their place as valuable tools for synthetic organic chemistry.[2] Since most of these transformations have been shown to occur with high regio- and/or stereoselectivity under very mild conditions, we were led to explore the possibility of exploiting these properties in preparing polyesters, which might not be accessible by more traditional methods of polycondensation, using enzyme-catalyzed transesterification. Among the advantages we expected to realize were the following: (1) It might be possibile to retain reactive functional groups in the polymer for use in subsequent manipulations. (2) It might be possible to construct polymers having highly regular structures by capitalizing on the regioand stereoselectivity of the enzyme. (3) The mild conditions of the polymerization might allow the molecular weight distribution (polydispersity) in the polymer to be controlled. (4) Polymers prepared using biocatalysts should be biodegradable. (5) The enzymes might provide models for synthetic catalysts having similar catalytic properties. Except for naturally occurring polyesters such as poly(13-hydroxybutyrate) and its copolymers with poly(P-hydroxyvalerate), there appears to have been limited previous effort to prepare either achiral or optically active polyesters using enzymes.[3] The polymerizations attempted generally led to small oligomers, and included AA + BB examples, A-B examples and one example of a polycarbonate formation
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